Electronic dryer circuit with pulsed sensing means



July 30, 1968 D. E. JANKE 3,394,457

ELECTRONIC DRYER CIRCUIT WITH PULSED SENSING MEANS Filed June 27. 1966 3Sheets-Sheet 1 FIG. I

D. E. JANKE July 30, 1968 ELECTRONIC DRYER CIRCUIT WITH PULSED SENSINGMEANS Filed June 27, 1966 3 Sheets-Sheet 2 INVENTOR.

DONALD E. JANKE BY #44, m f M Nor ATTORNEYS D- E. JANKE July 30, 1968ELECTRONIC DRYER CIRCUIT WITH PULSED SENSING MEANS Filed June 27, 1966 3Sheets-Sheet 5 TIME SHUT OFF TIME VOLTAGE 1 VOLTAGE INVENTOR.

DONALD E. JANKE United States Patent 3,394,467 ELECTRONIC DRYER CIRCUITWITH PULSED SENSING MEANS Donald E. Janke, Benton Harbor, Mich, assignorto Whirlpool Corporation, Benton Harbor, Mich, a corporation of DelawareFiled June 27, 1966, Ser. No. 560,420 9 Claims. (Cl. 34-45) ABSTRACT OFTHE DISCLOSURE A clothes dryer control circuit in which a DC signalcontaining an AC ripple energizes a relay coil whose closed contactsmaintain the dryer in operation. The same signal is also supplied to theparallel combination of a storage capacitor and a resistance whose valueis inversely proportional to the moisture content of the clothes. Whenthe clothes are wet the capacitor cannot charge due to the low value ofthe resistance. When the clothes become dry, however, the capacitorbegins to charge and its level is sensed by a threshold device sampledby the AC ripple. When the threshold device is triggered the capacitoris discharged through the relay coil in opposition to its holdingcurrent, thereby de-energizing the coil and terminating the dryeroperation.

This invention relates in general to an electronic control circuit for afabric conditioning apparatus, and more particularly to a novel clothesdryer control circuit featuring pulse sampled threshold sensing meansfor ensuring proper operation.

For many years the accepted method of terminating clothes dryer cycleswas by means of mechanical timers. Although in most cases the timerscould be pre-set to selected cycle lengths, such selection was asomewhat arbitrary one based primarily on the operators experience. As aresult, most drying cycles were terminated either too soon or too late,with remnant dampness or excessive shrinkage the undesirableconsequences.

More recently it has been proposed to effect drying cycle terminationthrough the use of control means responsive to the moisture content ofthe fabrics being treated, thus assuring shutdown at the optimum time toboth prolong fabric life and avoid power waste. In one prior art dryercontrol circuit a charging current is supplied to the parallelcombination of a storage capacitor and a moisture sensing means, thelatter consisting of a series of spaced conductive bands located on theinner surface of the dryer drum. When the clothes being treated aremoist, they complete a circuit path between the bands and shunt thecharging current to ground. As the clothes become drier their electricalresistance increases and a charge begins to build up on the capacitor.The magnitude of this charge is sensed by a threshold device in the formof a glow tube and, when the capacitor charge reaches a sufficientlevel, the tube fires and terminates the drying cycle.

Unfortunately, most voltage threshold sensing devices, including glowtubes and neon lamps, exhibit considerable inherent leakage current atvoltage levels just below their firing potentials. As a result of thischaracteristic, a danger exists in the prior art circuit just describedthat as the capacitor charge approaches the firing potential of thethreshold sensor, the leakage current through the latter will increaseto such an extent that the capacitor will never charge up to the firingpotential level. Under these conditions the threshold device will neverfire and the machine will enter into an endless drying cycle.

It is therefore a primary object of this invention to pro- 3,394,467Patented July 30, 1968 'ice vide a control circuit for a fabric dryingapparatus which overcomes the above and other disadvantages of the priorart systems by employing a pulse sampling technique to ensure properoperation.

It is a further object of this invention to provide such a controlcircuit which includes voltage regulating means to obviate the eifectsof power source fluctuations, and in which the voltage regulating meansinherently produces cyclic ripples that are converted to voltage pulsesand utilized to advantage in implementing the sampling technique.

It is a further object of this invention to provide such a controlcircuit which includes coil operated switch means to effect control ofthe operation of the clothes dryer and in which an initial currenttransient closes the coil operated switch means to commence the dryingcycle, a holding current in one direction through the switch coilmaintains the switch means closed throughout the drying cycle and acurrent flow through the switch coil opposite to the holding currentsubstantially reduces the effective current through the coil to open theswitch means and terminate the drying cycle when the dryness of theclothing has reached a pre-selected level.

It is a further object of this invention to provide such a controlcircuit in which the switch means may be connected in parallel with thevoltage regulating means whereby the opening of the switch means doesnot interrupt the availability of regulated voltage for other machinefunctions.

Briefly, according to the invention a regulated DC voltage containingcyclic ripples is supplied to the operating circuit of a reed switchcoil. The operating circuit contains a capacitor which converts theripples into voltage pulses and superimposes them on the DC voltagebeing applied to the coil. The regulated signal is also supplied to theparallel combination of the storage capacitor and a moisture responsiveresistance, the storage capacitor being connected in series with thereed switch coil. Parallel electrodes or conductive bands are spacedfrom each other and disposed on the inner surface of a dryer drum. Thedamp clothing in the drum lays against the bands and bridges the spacebetween them, thus providing an electrical resistance which is afunction of the moisture content of the clothing. The regulated currentis initially shunted to ground through the clothing. As they becomedrier and their resistance increases, however, a charge begins to buildup on the storage capacitor. The magnitude of this charge is sensed by aneon lamp connected across the storage capacitor and the reed switchcoil, so that the lamp actually sees the combined voltages across thecapacitor and the coil. Since the coil voltage contains cyclic pulsesthe lamp is thus effectively sampled at the cycle frequency rate at thepulses drive the lamp voltage up and down along its characteristiccurve. As the voltage across the capacitor increases the lamp will bedriven in and out of its current leakage region by the pulses, but sincetheir duration is relatively short compared with the cycle time, the netleakage will be small and the capacitor will continue to charge. Whenthe clothes are completely dry and the charge on the capacitor hasreached a sufficient level, one of the pulses will drive the lampvoltage above the firing potential and the lam will ionize. Thecapacitor will then discharge through the lamp and the reed switch coil.The discharge current through the coil is in opposition to its holdingcurrent which results in a net current in the coil that is substantiallybelow the reed switch drop out level and thereby ensures a positiveopening of the reed switch. This initiates a chain of circuit operationsthat terminate the drying cycle.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings, in which:

FIGURE 1 shows a perspective view of a clothes dryer including acut-away portion for illustrating the mounting arrangement for theconductive bands,

FIGURE 2 shows a schematic diagram of a dryer control circuitconstructed in accordance with the teachings of this invention,

FIGURE 3 shows a schematic diagram of a modification of a portion of thecircuit shown in FIGURE 2,

FIGURE 4 shows a time plot of the current waveform in the reed switchcoil,

FIGURE 5 shows the voltage versus current characteristic of the neonlamp, and

FIGURES 6 and 7 show time plots of the voltage waveforms at selectedpoints in the circuit of FIGURE 2.

As may be seen in FIGURE 1, a clothes dryer 10 according to thisinvention includes a rotating drum 11 having at least two electricallyconductive bands 12 and 14 mounted therein parallel to each other andspaced apart on an insulating pad 15, the latter being secured to theinner surface of the drum.

Referring now to FIGURES 2 and 3, rather than unduly burden thisspecification with a detailed and lengthy description of theinterconnections between each circuit element shown, which are believedto be obvious from the face of the drawings, such elements are insteadsimply listed below along with the corresponding reference numerals usedthroughout the drawings to designate them.

12, 14conductive bands (electrodes) 16dryer motor Iii-run winding20start winding 22--heating element 24-door operated switch 26, 28motoroperated centrifugal switches 30spring biased start switch (normallyopen) 32--thermostatic switch (opens on cooling) 34--relay coil 36,38-relay operated switches (normally open) 42--limiting resistor 44reedswitch 46-diode rectifier 48, 50resistors 52-filter capacitor 54Zenerdiode 56-dilTerentiating capacitor 58-resistor 60--reed switch coil62diode 63-variable resistor 64storage capacitor 66, 68limitingresistors 70neon lamp 72electronic control section Turning now to theoperation of the dryer circuit shown in FIGURE 2, which will beexplained in conjunction with the voltage and current waveforms shown inFIGURES 4-7, 220-volt, 60-cycle power is supplied to the system atterminals L1, L2 and N, the latter being neutral or ground. Assumingthat wet clothes have been loaded into the dryer drum and that doorswitch 24 has been closed by the closing of the dryer door, thedepression of start switch 30 causes the sinusoidal waveform V shown inFIGURE 7 to appear at point A. This waveform energizes relay coil 34which closes its associated switches 36 and 38. The closing of switch 36supplies power to both run winding 18 and start winding 20 of dryermotor 16. When the motor comes up to speed, centrifugal switch 26transfers to its other position and centrifugal switch 28 closes. Thetransfer of switch 26 de energizes the start winding 20 while theclosing of switch 28 completes the circuit path through heating element22 and previously closed switch 38, and the heating element is thusenergized by the full 220 volt line voltage appearing across terminalsL1 and L2. It is to be understood that while an electric heating elementhas been disclosed, this invention is equally applicable to any type ofdrying apparatus, and element 22 could be replaced by an electricallyoperated valve in the case of a gas dryer.

When the start switch 30 is depressed, the alternating signal at point Aalso enters the electronic control section 72. This signal is half-waverectified by diode 46 and smoothed by the filter network comprisingresistor 48 and capacitor 52, with the resulting DC waveform at point Bshown as V in FIGURE 7. This waveform is applied through resistor 50 toZener diode 54, which performs a regulating function by reason of itsreverse breakdown characteristic to produce waveform V in FIGURE 7 atpoint C. It will be noted that the voltage V is not a straight linefunction, but contains a small negative ripple during each cycle.

The initial transient created at point C by the depression of the startswitch surges through capacitor 56 and the impedance afforded by coil60. As seen in FIGURE 4, the magnitude of this transient current exceedsthe pull-in threshold and reed switch 44 closes. As capacitor 52stabilizes, the transient decays. The DC signal at point C is blocked bycapacitor 56, but resistor 58 passes sufiieient holding current tomaintain the reed switch in its closed position, as shown by the runportion of the curve in FIGURE 4. Capacitor 56, however, does pass thecyclic ripples in waveform V and superimposes them on the DC signalsupplied by resistor 58, thereby resulting in waveform V of FIGURE 6 atpoint D. Since the reed switch 44 is connected in parallel with thestart switch 30 through centrifugal switch 26, it acts as a holdingcircuit when closed, and all of the circuit elements continue inoperation when the start switch is released. When the dryer temperaturereaches a sufficient level, thermostatic switch 32 closes, whichprovides an alternate line signal path through motor 16 and electroniccontrol section 72. Closed reed switches exhibit very low contactresistance and because of this, limiting resistor 42 is connected inseries with reed switch 44 to preclude the possibility of motor 16 beingsupplied current through the circuit which includes reed switch 44.

The DC signal at point C is also applied through variable resistor 63 tostorage capacitor 64, neon lamp and conductive band 12. As long as theclothing in the dryer drum is wet the current available at point E isshunted to ground across bands 12 and 14 through the low resistance pathoffered by the clothing. As the clothing becomes drier, however, itsresistance increases and a charge gradually builds up on capacitor 64,as shown by curve V in FIGURE 6. The charging rate of the capacitor maybe controlled by the adjustment of variable resistor 63. The voltageimpressed upon neon lamp 70, which sees the combined voltages acrossstorage capacitor 64 and reed switch coil 60, also begins to increase asshown by curve V in FIGURE 6. This curve represents the summation ofcurves V and V As the lamp voltage increases, the current leakage regionis approached, as shown in the characteristic curve of the lamp inFIGURE 5. If the lamp was gradually driven into the leakage region bymonitoring only the voltage across storage capacitor 64, a distinctpossibility exists that the leakage current would be sufficient toprevent the capacitor from ever charging above the firing potential V Inthis event the dryer would enter an undesirable endless run cycle.

Since the lamp 70 in the instant circuit sees the cyclic pulse signals74, as well, however, whose magnitudes are greater than the differencebetween V and V in FIG- URE 5, the lamp voltage is only driven into theleakage region during the brief sampling times afforded by the pulses.As a result, the current leakage is minimized and the storage capacitor64 continues to charge during the relatively long intervals between thepulses.

When the clothes become dry and the capacitor charge builds up to asufficient level, one of the sampling pulse signals 74 drives the lampvoltage above the firing threshold V and the lamp ionizes. Capacitor 64then discharges through the lamp and the reed switch coil 60. Thedischarge current through the coil is in a direction opposite to that ofthe holding current through resistor 58 which substantially reduces thenet current through the coil. In one embodiment the magnitude of thedischarge current was sufficient to cause a complete polarity reversalacross the coil, as shown by the initial shut-off portion of the curvein FIGURE 4. Reed switch 44 now opens, which de-energizes both the relaycoil 34 and the electronic control section 72. When coil 34 isde-energized, its associated switches 36 and 38 return to their openpositions. The opening of switch 38 de-energizes the heating element 22,but the opening of switch 36 has no immediate effect since the runwinding 18 of the dryer motor 16 is still supplied through the closedthermostatic switch 32 and the transferred centrifugal switch 26. Whenthe dryer cools down sufliciently thermostatic switch 32 opens and motor16 is tie-energized, thus terminating the entire drying cycle.

When the capacitor 64 discharges to the point where the voltage at pointB is below the conduction maintaining potential of the lamp V the lampextinguishes and reverts to its non-conductive state. By this time thereed switch 44 has already opened, however, and the decaying voltage atpoint C soon reaches a lower level than that at point E. When thishappens the diode 62, which was reverse biased during the entire startand run cycles, becomes forward biased and any charge remaining on oneside of the capacitor 64 is immediately passed to the opposite sidethrough the diode 62 and resistor 58. This prevents an operator fromreceiving an electrical shock if he touches the conductive band 12 whileunloading the clothes from the dryer. As an additional safety feature,resistors 66 and 68 are incorporated into the circuit as shown to limitany current with which an operator might come into contact.

In the modified portion of the circuit shown in FIG- URE 3, reed switch44 is connected in series only with relay coil 34 rather than in serieswith both the relay coil 34 and control section 72 as is the case inFIGURE 2. The effect of this change is that when reed switch 44- opensin response to the lamp ionization and deenergizes relay coil 34,regulated DC power is still available at point C within electroniccontrol section 72, until thermostatic switch 32 opens. Since capacitor52 is stable, no initial transient surge is again developed throughcapacitor 56 and reed switch coil 60. Therefore, the reed switchcontacts will remain open, and, although the control section 72 is stillenergized, it is ineffective to exercise any control functions. Theadvantage of this modification is that DC power is still available untilthermostatic switch 32 opens for the operation of auxiliary dryercomponents, such as a lint indicator or shut-off alarm.

It can thus be seen that this invention provides a rnoisture responsivedryer control circuit that employs a novel pulse sampling technique toensure proper and reliable operation. This is accomplished at noincrease in circuit complexity since the pulse generating element,capacitor 56, is already in the circuit to provide a pull-in currentsurge for operating the reed switch coil, and the sampling pulses arethus obtained free.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. In a clothes dryer having means defining a drying chamber forreceiving wet fabrics to be dried, electrode means in said dryingchamber for sensing the resistance of said fabrics, an AC power source,heating means connected to said AC power source for supplying heat tosaid drying chamber, and a control circuit connected to said heatingmeans and to said electrode means for terminating the operation of saidheating means in response to a resistance of said fabrics across saidelectrode means indicative of a dried condition thereof, said controlcircuit comprising: a storage capacitor connected in parallel with saidelectrode means; impedance means in series with said storage capacitor;selectively conductive threshold sensing means having a firing potentialand an inherent current leakage region within the nonconductive statethereof, said threshold sensing means being conductive when the combinedvoltage across said storage capacitor and said impedance means reachessaid firing potential; means responsive to the conduction of saidthreshold sensing means for terminating operation of said heating means;DC power supply means connected to said AC power source for supplying acharging current to said storage capacitor, a charge on said storagecapacitor increasing toward the firing potential of said thresholdsensing means as the resistance of said fabrics across said electrodemeans increases, and means connected to said power supply means forapplying a cyclic pulse signal across said impedance means whereby saidcyclic pulse signal drives the threshold sensing means into and out ofsaid current leakage region to minimize current leakage through saidthreshold sensing means during the noncond-uctive state thereof.

2. The apparatus of claim 1 wherein said DC power supply means providesa DC signal having a cyclic ripple, and wherein the means for applying acyclic pulse signal across said impedance means includes adifferentiating capacitor connected to said impedance means forconverting said cyclic ripple into a cyclic pulse signal.

3. The apparatus of claim 1 wherein the threshold sensing means is aneon lamp.

4. The apparatus of claim 1 wherein said impedance means is a reedswitch coil, wherein the charging current supplied by said DC powersupply is also supplied to said reed switch coil in a first direction toact as a holding current, wherein said threshold sensing means isconnected across the series combination of said storage capacitor andsaid reed switch coil, and wherein said control circuit further includesa reed switch operatively associated with said reed switch coil forsupplying electrical power to said power supply means from said AC powersource when closed, whereby the conduction of said threshold sensingmeans discharges said storage capacitor through said reed switch coil ina second direction opposite to said first direction to thereby reducethe current flow through said reed switch coil to open said reed switchand terminate the operation of said heating means.

5. The apparatus of claim 4 wherein said DC power supply means includesvoltage rectifying, filtering and regulating means for converting analternating signal from said AC power source to a DC signal having acyclic ripple, said DC power supply means connected in series with saidreed switch.

6. The apparatus of claim 1 wherein said impedance means is a reedswitch coil, wherein the charging current supplied by said power supplymeans is also supplied to said reed switch coil in a first direction toact as a holding current, wherein said threshold sensing means isconnected across the series combination of said storage capacitor andsaid reed switch coil, wherein the means responsive to the conduction ofsaid threshold sensing means for terminating the operation of saidheating means includes a relay coil, and wherein said control circuitfurther includes a reed switch operatively associated with said reedswitch coil for controlling the energization of said relay coil fromsaid AC power source, whereby the conduction of said threshold sensingmeans discharges the storage capacitor through said reed switch coil ina second direction opposite to said first direction, thereby reducingthe current fiow through said reed switch coil to open said reed switchand terminate the operation of said heating means.

7. The apparatus of claim 6 wherein said reed switch is connected inseries with said relay coil, and wherein said power supply means isconnected in parallel with said series combination of said reed switchand said relay coil whereby said power supply means continues to operatefrom said AC power source subsequent to the opening of said reed switch.

8. In a clothes dryer having means defining a drying chamber forreceiving wet fabrics to be dried, electrode means in said dryingchamber for sensing the resistance of said fabrics, an AC power source,heating means connected to said AC power source for supplying heat tosaid drying chamber and a control circuit connected to said heatingmeans and to said electrode means for terminating the operation of saidheating means in response to a resistance of said fabrics across saidelectrode means indicative of a dried condition thereof, said controlcircuit comprising: a storage capacitor connected in parallel with saidelectrode means; a reed switch coil in series with said storagecapacitor; selectively conductive threshold sensing means having afiring potential, said threshold sensing means being conductive when thevoltage across said storage capacitor reaches said firing potential; arelay coil responsive to the conduction of said threshold sensing meansfor terminating the operation of said heating means; DC power supplymeans connected to said AC power source for supplying a charging currentto said storage capacitor, the charge on said storage capacitorincreasing toward the firing potential of said threshold sensing meansas resistance of said fabrics across said electrode means increases,said power supply means further supplying a holding current in a firstdirection through said reed switch coil; and a reed switch operativelyassociated with said reed switch coil and connected in series with saidrelay coil for controlling the energization of said relay coil, wherebythe conduction of said threshold sensing means discharges the storagecapacitor through said reed switch coil in a second direction oppositeto said first direction to thereby reduce the current flow through saidreed switch coil to open said reed switch and terminate operation ofsaid heating means, said power supply means being connected in parallelwith said series combination of said reed switch and said relay coilwhereby said power supply means continues to operate from said AC powersource subsequent to the opening of said reed switch.

9. In a clothes dryer having means defining a drying chamber forreceiving wet fabrics to be dried, electrode means in said dryingchamber for sensing the resistance of said fabrics, an AC power source,heating means connected to said AC power source for supplying heat tosaid drying chamber, and a control circuit connected to said heatingmeans and to said electrode means for terminating the operation of saidheating means in response to a resistance of said fabrics across saidelectrode means indicative of a dried condition thereof, said controlcircuit comprising; a storage capacitor connected 'in parallel with saidelectrode means; a reed switch coil in series with said storagecapacitor; selectively conductive threshold sensing means having afiring potential, said threshold sensing means being conductive when thevoltage across said storage capacitor reaches said firing potential; arelay coil responsive to the conduction of said threshold sensing meansfor terminating the operation of said heating means; a reed switchoperatively associated with said reed switch coil for controlling theenergization of said relay coil from said AC power source; a startswitch for initiating operation of said clothes dryer, said start switchproviding an instantaneous line transient upon closing; adifferentiating capacitor in series with said reed switch coil whichpasses said line transient to said reed switch coil in a first directionfor closing said reed switch and initiating operation of said heatingmeans; and DC power supply means connected to said AC power source forsupplying a holding current through said reed switch coil in said firstdirection, said power supply means further supplying a charging currentto said storage capacitor, the charge on said storage capacitorincreasing toward the firing potential of said threshold sensing meansas the resistance of said fabrics across said electrode means increases,whereby the conduction of said threshold sensing means discharges thestorage capacitor through said reed switch coil in a second directionopposite to said first direction to thereby reduce the current flowthrough said reed switch coil to open said reed switch and terminateoperation of said heating means.

References Cited A. D. HERRMANN, Assistant Examiner.

